For the purposes of the present discussion, a free space optical link is defined to be any transmission path in which data is communicated by modulating a light beam that travels through space to a detector. Free space optical links are being using with increasing frequency for the transmission of data. For example, infra-red optical ports for communicating data from a portable computer to a docking station or other computer are well known in the art. This type of link is also used in remote control units for operating television sets, VCR's, and the like.
The light entering the photodetector is the sum of the ambient background light and the modulated light beam from the transmitter. The most common background light sources are sunlight or fluorescent lighting. These background sources contribute a significant DC photocurrent to the output of the detector. Additionally, some background sources may contribute signals within the frequency range of the transmitted signal.
Detected DC background signals can cause problems in two ways. First, there is a type of noise known as shot noise which is proportional to the square root of the detected power. If the DC background signal is much larger than the data signal of interest, the shot noise from the DC background can become the dominant noise source and thus limit the minimum detectable signal power. Since the total shot noise power is related to the detection bandwidth, the problem becomes more significant at higher data rates. Furthermore, since DC background signals are often much larger than the data signal, the saturation level of a receiver with a DC coupled front end (such as a trans-impedance amplifier) must be designed to accommodate them. Because of the finite dynamic range of a receiver, this in turn can limit minimum detectable signal power.
As the data rate that these links are required to accommodate increases, problems caused by background light sources become more significant. As the bit rate increases, the power per bit decreases unless the intensity of the light source that is modulated in the transmitter can be increased. The decreased power per bit leads to an increase in the error rate unless the background light is also reduced.
There is a limit on the light intensity that can be utilized to transmit data. The transmitters of choice are lasers. Hence, the need to prevent eye damage in the event a transmitter is inadvertently aimed at the eye limits the maximum power of the light source. Accordingly, any improvements in signal to noise ratio must come from reducing the noise or increasing the dynamic range.
Two approaches have been suggested to reduce the errors caused by the background light. The first approach is based on the assumption that the background light intensity is constant in time. This approach utilizes front-end circuitry at the detector to subtract off any DC photocurrent from the front-end amplifier. A detector with capacitive coupling to the receiver circuit will also address this problem, but is rarely used since DC coupled front end circuits, such as a trans-impedance amplifier, generally provide much higher performance. This approach fails in situations in which the background light has high frequency components within the frequency band of the data signal. For example, background light from video displays may vary rapidly in time depending on the scene being displayed. Additionally, some forms of fluorescent lighting generate high frequency optical signals.
The second method utilizes the wavelength of the transmitter signal to distinguish the background light from the carrier light signal. This approach reduces the background noise by utilizing inexpensive wavelength filters made from photographic film to remove the portion of the background light that is outside the frequency band of the transmitter. However, the bandpass of the wavelength filter must still be large compared to the bandwidth of a laser to assure an economical filter design. Hence, systems based on wavelength filters are still subject to significant background light interference from sources that emit within the pass band of the filter.
Broadly, it is the object of the present invention to provide an improved optical transmission system.
It is a further object of the present invention to provide an optical transmission system, which is more resistant to background light than prior art systems.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.